275 kV

thumbnail photo of L6 pylon

Larger pylon (L6)

thumbnail photo of transmission pylon

Smaller pylon (L2)

 

275 kV lines are built to the same designs as 400 kV lines and can carry the same loads so they produce the same magnetic fields. (To be strictly correct, a 275 kV line can be 7.3 m instead of 7.6 m above ground, but that has been ignored here.) They produce lower electric fields because of the lower voltage.

Magnetic fields

The maximum field is produced by the largest design of line – the L6 – when the ground clearance is the minimum allowed – 7.6 m – and the loads are the highest allowed – 4.7 kA in each circuit.  The field also depends on the phasing. Transposed (T) is higher close to the line and Untransposed (U) is higher away from the line.  This graph shows both.

 graph of maximum field from 275 kV line

Typical fields are lower than the maximum field because the clearance is usually higher and the loads are usually lower.  Also, many lines are a slightly smaller design – the L2.  Both L6 and L2 are shown on the following graph.  The phasing here is T as this is more common. It is rare for the two circuits to carry exactly the same load.  That is why the magnetic field shown here is not symmetrical.

graph of typical field from 275 kV line 


This table gives some actual field values for the same conditions.

     

magnetic field in µT at distance from centreline

maximum under line

10 m

25 m

50 m

100 m

275 kV

largest lines

L6
quad bundles
0.305 m
zebra

maximum

clearance 7.6 m
phasing U
load 4.7/4.7 kA

108.422

95.780

38.422

11.697

3.096

typical

clearance 13 m
phasing T
load 0.4/0.6 kA

5.783

5.247

2.194

0.578

0.119

smaller lines

L2
twin bundles
0.305 m
zebra

maximum

clearance 7.6 m
phasing U
load 2.4/2.4 kA

54.142

46.300

16.283

4.865

1.278

typical

clearance 13 m
phasing T
load 0.4/0.6 kA

4.971

4.158

1.557

0.400

0.084

typical design used for new lines

L12
twin bundles
0.5 m
araucaria

maximum

clearance 7.6 m
phasing U
load 3.5/3.5 kA

81.942

72.818

22.103

8.148

2.145

typical

clearance 13 m
phasing T
load 0.4/0.6 kA

5.604

4.938

1.979

0.514

0.106

Note:

1. All fields calculated at 1 m above ground level.

2. All fields are given to the same resolution for simplicity of presentation (1 nT = 0.001 µT) but are not accurate to better than a few percent.

3. Calculations ignore zero-sequence current.  This means values at larger distances are probably underestimates, but this is unlikely to amount to more than a few percent and less closer to the line.

4. The “maximum field under the line” is the largest field, which is not necessarily on the route centreline; it is often under one of the conductor bundles.

Electric fields

The maximum field is produced by the largest design of line – the L6 – when the ground clearance is the minimum allowed – 7.6 m. The field also depends on the phasing. Untransposed (U) is generally higher and is shown here.

graph showing maximum electric field from 275 kV 

Typical fields are lower than the maximum field because the clearance is usually higher.  Also, many lines are a slightly smaller design – the L2.  Both L6 and L2 are shown on the following graph.  The phasing here is T as this is more common.

graph showing typical electric field from 275 kV

This table gives some actual field values for the same conditions.

     

electric field in V m-1 at distance from centreline

maximum under line

10 m

25 m

50 m

100 m

275 kV

largest lines

L6
quad bundles
0.305 m
zebra

maximum

clearance 7.6 m
phasing U

7838

6964

532

307

107

typical

clearance 13 m
phasing T

2918

2892

661

73

22

smaller lines

L2
twin bundles
0.305 m
zebra

maximum

clearance 7.6 m
phasing U

6804

4550

195

245

81

typical

clearance 13 m
phasing T

2151

2038

322

39

19

typical design used for new lines

L12
twin bundles
0.5 m
araucaria

maximum

clearance 7.6 m
phasing U

7316

5782

460

278

94

typical

clearance 13 m
phasing T

2547

2472

503

67

18

Note:

1. All fields calculated at 1 m above ground level.

2. All electric fields are calculated for the nominal voltage.  In practice, voltages (and hence fields) may rise by a few percent.

3. All electric fields calculated here are unperturbed values.

4. All fields are given to the same resolution for simplicity of presentation (1 V/m) but are not accurate to better than a few percent.

5. Calculations ignore zero-sequence voltages.  This means values at larger distances are probably underestimates, but this is unlikely to amount to more than a few percent and less closer to the line.

6. The “maximum field under the line” is the largest field, which is not necessarily on the route centreline; it is often under one of the conductor bundles.

Underground cables

275 kV underground cables are built in the same way as 400 kV cables so produce the same fields.

Three main types of underground cable are used.

  • Trough: the separate cores of the cable are in a concrete trough, typically only 0.3 m or less below ground, but also only 0.15 m apart
  • Direct buried: the separate cores of the cable are laid directly in the ground, typically 1 m below ground and 0.3-0.5 m apart
  • Tunnel: the cable is carried in a tunnel typically 20 m below ground
     

See photos and diagrams of these different types of cable.

Cables in tunnels are so far below ground that the fields at ground level are usually below background levels.  Maximum fields from typical examples of the other two types are shown in this graph.  This is for 1 m above ground.  At this height, the direct-buried cable produces the higher field.  Closer to the ground, the trough produces the higher field. Fields from underground cables are very sensitive to the height above ground. more detail

graph of maximum field 275 kV underground 

Typical fields are lower than the maximum field because the loads are usually lower.  Typical fields are shown in the following graph.

graph of typical field 275 kV underground 

Underground cables do not produce any external electric fields.

Obtaining a higher rating with an underground cable can involve installing multiple groups of conductors - see how this affects the magnetic field.

This table gives some actual field values for the same conditions.

    

magnetic field in µT at distance from centreline

0 m

5 m

10 m

20 m

275 kV

trough

0.13 m spacing 0.3 m depth

maximum

83.30

7.01

1.82

0.46

typical

20.83

1.75

0.46

0.12

direct buried

0.5 m spacing 0.9 m depth

maximum

96.17

13.05

3.58

0.92

typical

24.06

3.26

0.90

0.23

Notes

1. All fields calculated at 1 m above ground level

2. All fields are given to the same resolution for simplicity of presentation (0.01 µT = 10 nT) but are not accurate to better than a few percent.

3. Calculations ignore zero-sequence current.  This means values at larger distances are probably underestimates, but this is unlikely to amount to more than a few percent.

4. Cable designs are not standardised to the same extent as overhead lines and the examples given here are representative.

5. The trough calculation is for a double circuit and the direct buried is for a single circuit, but in practice there may be other nearby circuits which affect the field.